Inline and real-time microfluidic relative permittivity sensor using highly doped silicon sidewall electrodes

This paper reports an inline sensor for the real-time measurement of the relative permittivity of a continuously flowing fluid, with electrodes that are electrically isolated from the fluid. The sensor has been characterised for relative permittivity values ranging from 1 to 80 - including water and water-containing mixtures - showing an accuracy within 3% of full scale. The proposed sensor contains multiple parallel microchannels, each of which has parallel electrodes in the sidewall, resulting in a low pressure drop and high sensitivity while the internal volume is only 50 nL. The fabrication technology is scalable: equally deep microchannels with widths of 5, 10, 20, and 40 mu m are demonstrated. The technology is also compatible with other inline sensors, enabling further on-chip integration.